To quote Book-a-Minute, Five billion people almost DIE, and it is FUNNY.
That's the gist of Good Omens. But let's forget about the five billion almost dying, and focus on the funny. Because it's funny. Brilliantly so. Hilarious, actually.
It may sound obvious, but think about Terry Pratchett and Neil Gaiman, and put them together. Terry Pratchett's humour and Neil Gaiman's knack for surreality. That's all in there, tied together so well, that it is impossible to tell them apart.
Good Omens is about the Apocalypse and its Four Horsemen, the Anti-Christ, The Beginning and End of Things, Angels and Demons and everything else that comes with it. But most of all it is a set of stories that all converge into one point. The characters are believable, they are surreal, and you do expect them to pop out in the your street at any time, even if it is The Antichrist.
This book is a favourite of mine. When I need something that will amuse me and provide a good read this is usually the book I pick up. If I am to be found chuckling or even laughing out loud alone, this book is sure to be the cause of it.